This invention relates to transverse discharge devices and particularly to such devices for use with lasers.
Discharge devices used in excimer gas lasers have had two slab electrodes connected to a power supply by means of two separate current carrying members disposed outside the device housing containing the slab electrodes and the discharge region between the electrodes. One member runs from one device electrode down along both sides of the device while another member runs from another device electrode with both members running beyond the last mentioned electrode. This device geometry involves power feed to the electrodes along a path that has an adverse effect on an electric field formed between the device electrodes in the discharge region. Such a device is called a transverse discharge device because the discharge between the electrodes is transverse to the device's optical axis.
Transverse discharge devices, particularly those incorporated into gas lasers, have an inductance that can degrade laser performance. Past efforts to minimize this inductance have involved making the device walls enclosing the discharge region as close as possible to the edges of the opposed slab electrodes separated by the length of preferred discharge path in the region. However, mere proximity of any inside wall surface to the discharge suffices to cause a flashover of an initiated discharge to such surface and tracking of the discharge along such wall surface to a discharge electrode; the discharge thus follows a route other than a path through the discharge region from one electrode to another. This process limits the compactness of the device. Various attempts have been made to overcome this problem with only limited success.
Prior attempts to overcome the problem of discharge tracking involved designing devices to make the wall track length an arbitrarily large multiple of the preferred discharge path length. In small volume laser discharge devices (for example, in excimer lasers) this design approach works and poses no new problem inasmuch as the devices being small sized have inductance that remains acceptably low. However, as the volume of the laser discharge region increases in size, above about 3 liters, device inductance rapidly increases to become so great as to impair efficient laser operation. Techniques utilized heretofore in these prior attempts to overcome the problem maintained the discharge track length at some multiple of the length of the preferred discharge path. In a Xenon Chloride (XeCl) laser discharge device of most common geometry this multiple has been empirically found to be 3.
Other techniques involve grooving or serrating the wall surface whereon unwanted discharge tracking ordinarily takes place, to increase track length. Yet another technique involves anti-tracking bars affixed to the wall surface, again, to increase track length. Both techniques are limited, although useful in certain devices, and do not provide entirely predictable results. Antitracking bars in particular are very limited in utility, due to a tendency of an initiated discharge to flashover from a point of origination in a discharge region at or near a high potential electrode to the bars, then travel to a ground electrode by tracking along the wall surface(s). "Punch through" can occur if the bars are not tightly sealed to the wall and defeats the bars' purpose as discharge tracking occurs on the wall under the bars. Grooved walls, likewise, suffer a similar drawback; that is, a tendency of a discharge to flashover from an electrode edge to a wall surface and then track. This is evidence that increased track length by itself is inadequate to inhibit problemsome discharge tracking, unless the wall surface is kept at a distance far from an electrode edge. As this distance increases, the problem of increased device inductance reappears.
The above techniques can be contrasted with electric field shaping techniques utilized recently in various devices One device design utilizes metal vanes outside a cylindrical housing to modify an electric field in a discharge region between two slab electrodes. These vanes flank both sides of a housing of dielectric material, such as plastic, and helps shape the electric field in the region. Without this field modification an extra high intensity field exists in the space surrounding both long edges of the driven electrode. The field intensity in this space is much higher than the average field intensity in the rest of the discharge region. Analysis indicates that such "enhancement" of field intensity in a localized portion of the field will cause a discharge corona to occur from which a discharge will travel from an electrode edge to the laterally adjacent wall surface. The direction of the localized high field will accelerate any charged particle, particularly a negatively-charged electron, toward the wall surface, also leading to undesired discharge tracking. When the shield is fitted onto the housing the shield acts electrostatically on the discharge region, so that the potential difference between the electrode and the nearby wall is eliminated, because the shield is electrically connected to the electrode. However, even this shield does not entirely eliminate unwanted field enhancements. Argon gas put into the discharge region shows the unwanted discharge tracking even though a XeCl mix (a mixture of Xe, HCl, and Ne) in the discharge region is free of discharge tracking. Argon is most susceptible to discharge tracking and poses a severe test of this device design. Thus, reducing the electric field that initiates a discharge has merit, yet is not, in and of itself, entirely successful in avoiding the tracking phenomenon. Control of electric field intensity and gradient throughout all field portions should be a design criteria, if electrostatic shielding is to solve the problem.